This invention relates to polyurethanes, to inks comprising polyurethanes and to their use in ink jet printing.
Ink jet printing involves printing an image onto a substrate by ejecting ink droplets through a fine nozzle onto the substrate without bringing the fine nozzle into contact with the substrate.
There are many demanding performance requirements for colorants and inks used in ink jet printing. For example they desirably provide sharp, non-feathered images having good water-fastness, light-fastness and optical density. The inks are often required to dry quickly when applied to a substrate to prevent smudging, but they should not form a crust which would block the tip of the fine nozzle. The inks should also be stable to storage over time without decomposing or forming a precipitate which could also block the fine nozzle. Thermal and piezoelectric ink jet printers are widely used, thus there is a need for inks, suitable for use in both types of printers, having high colour strength and giving images having light fastness when printed on a typical substrate, especially plain paper.
EP 0769 509 describes a high molecular weight chain extended polyurethane, formed from a colored isocyanate-terminated polyurethane prepolymer, for use in ink jet printers with piezo heads. However this composition is not ideal for use in ink jet printers with thermal heads because the use of heat can result in nozzle blockage and other operability problems.
According to a first aspect of the present invention there is provided a non-isocyanate terminated, colored, water-dissipatable polyurethane having a weight average molecular weight less than 30,000 obtainable from the reaction of a mixture comprising the components:
i) at least one organic polyisocyanate;
ii) at least one isocyanate-reactive compound providing water-dispersing groups; and
iii) at least one colorant having at least two functional groups selected from isocyanate groups and isocyanate-reactive groups.
The colored water-dissipatable polyurethane preferably has a weight average molecular weight (Mw) less than 30,000 because this Mw leads to an improved performance of inks containing the polyurethane, especially for use in thermal ink jet printers. The Mw of the polyurethane is preferably less than 25,000, more preferably from 1000 to 25,000, more preferably from 1000 to 20,000, most preferably from 1000 to 15000 and especially from 1000 to 10,000. Mw may be measured by gel permeation chromatography.
The gel permeation chromatography method used for determining Mw preferably comprises applying the polyurethane to a chromatography column packed with cross-linked polystyrene/divinyl benzene, eluting the column with tetrahydrofuran at a temperature of 40xc2x0 C. and assessing the Mw of the polyurethane compared to a number of a polystyrene standards of a known Mw. Suitable chromatography columns packed with cross-linked polystyreneldivinyl benzene are commercially available from Polymer Laboratories.
As an alternative to the gpc method for determining Mw, one may use other methods, for example multi-angle light scattering.
Component i) may be any organic polyisocyanate known in the art, preferably having two isocyanate groups, and include aliphatic, cycloaliphatic, aromatic or araliphatic isocyanate. Examples of suitable organic polyisocyanates include ethylene diisocyanate, 1,6-hexamethylene diisocyanate, isophorone diisocyanate, tetramethylxylene diisocyanate, 1,4-phenylene diisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, 4,4xe2x80x2-diphenyl-methane diisocyanate and its hydrogenated derivative, 2,4xe2x80x2-diphenylmethane diisocyanate and its hydrogenated derivative, and 1,5-naphthylene diisocyanate. Mixtures of the polyisocyanates can be used, particularly isomeric mixtures of the toluene diisocyanates or isomeric mixtures of the diphenylmethane diisocyanates (or their hydrogenated derivatives), and also organic polyisocyanates which have been modified by the introduction of urethane, allophanate, urea, biuret, carbodiimide, uretonimine or isocyanurate residues.
Preferred organic polyisocyanates include cycloaliphatic polyisocyanates, especially isophorone diisocyanate, and aliphatic isocyanates, especially 1,6-hexamethylene diisocyanate or hydrogenated 4,4-diphenyl methyl diisocyanate. A small quantity of triisocyanates may be included as part of component i) but this amount preferably does not exceed 5% by weight relative to the total weight of component i). In a preferred embodiment component i) consists of a mixture of diisocyanate and from 0 to 5% of triisocyanate by weight relative to the diisocyanate.
Component ii) providing water-dispersing groups preferably has at least one, and preferably two, isocyanate-reactive groups. Preferred isocyanate-reactive groups are selected from xe2x80x94OH, xe2x80x94NH2, xe2x80x94NHxe2x80x94and xe2x80x94SH. Isocyanate-reactive compounds having three isocyanate-reactive groups may be present, preferably in low levels not exceeding 5% by weight relative to the total weight of component ii). These isocyanate-reactive groups are capable of reacting with an isocyanate (xe2x80x94NCO) group in component i) or component iii).
The water-dispersing groups are preferably present in the polyurethane as in-chain, pendant or terminal groups. Further water-dispersing groups may be introduced into the polyurethane by means of a capping reagent having one isocyanate or isocyanate-reactive group and a water-dispersing group.
The nature and level of water-dispersing groups in the polyurethane influences whether a solution, dispersion, emulsion or suspension is formed on dissipation of the polyurethane.
The water-dispersing group content of the polyurethane may vary within wide limits but is usually selected to be sufficient to ensure the polyurethane forms stable ink-jet printing inks in water and aqueous media. The polyurethane is preferably soluble in water, although minor amount of the polyurethane may be insoluble in water and exist as dispersed particles when mixed with aqueous media or water.
Preferably the proportion of insoluble polyurethane is less than 50%, preferably less than 40% and most preferably less than 30% by weight relative to the total weight of the polyurethane. This allows the preparation of concentrates which may be used to prepare more dilute inks and reduces the chance of the polyurethane precipitating if evaporation of the aqueous media occurs during storage. The size of insoluble polyurethane particulates when dissipated in an ink is preferably less than 100nm, and more preferably less than 60nm.
The water-dispersing groups may be ionic, non-ionic or a mixture of ionic and non-ionic water-dispersing groups. Preferred ionic water-dispersing groups include cationic quaternary ammonium groups and anionic phosphonic acid groups, sulphonic acid groups and carboxylic acid groups. More preferred ionic water-dispersing groups are sulphonic acid groups and carboxylic acid groups.
The ionic water-dispersing groups may be incorporated into the polyurethane in the form of a low molecular weight monomers, polyol or polyamine bearing the appropriate ionic water-dispersing groups where polyol excludes polyesters bearing sulphonic acid groups. Preferred isocyanate-reactive compounds providing water-dispersing groups are diols having one or more carboxylic acid groups or sulphonic acid groups, where examples include bis(2-hydroxyethyl)-5-sodiosulphoisophthalate and dihydroxy alkanoic acids, especially 2,2-dimethylol propionic acid and/or mixtures thereof.
The carboxylic and sulphonic acid groups may be subsequently fully or partially neutralised with a base containing a cationic charge to give a salt. If the carboxylic or sulphonic acid groups are used in combination with a non-ionic water-dispersing group, neutralisation may not be required. The conversion of any free acid groups into the corresponding salt may be effected during the preparation of the-polyurethane and/or during the preparation of an ink from the polyurethane.
Preferably the base used to neutralise any acid water-dispersing groups is ammonia, an amine or an inorganic base, more preferably ammonia is used. Suitable amines are tertiary amines, for example triethylamine or triethanolamine. Suitable inorganic bases include alkaline hydroxides and carbonates, for example lithium hydroxide, sodium hydroxide, or potassium hydroxide. A quaternary ammonium hydroxide, for example N+(CH3)4OHxe2x88x92, can also be used. Generally a base is used which gives the required counter ion desired for the ink which is prepared from the polyurethane. For example, suitable counter ions include Li+, Na+, K+, NH4+, Cs+ and substituted ammonium salts, including tributylammonium, imidazolium, tetramethyl ammonium, tetraethyl ammonium, tetrabutyl ammonium, tetrabutyl phosphonium and trimethyl sulphonium salts. NH4+ is especially preferred.
Prefered non-ionic water-dispersing groups are in-chain, terminal and pendant polyoxyalkylene groups, more preferably polyoxypropylene and polyoxyethylene groups. Examples include groups of the formula: RO(CH2CH2O)nH, RO(CH2CH2O)n(CH2CH(CH3)O)yCH2CH(CH3)NH2, or H2NCH(CH3)CH2(CH(CH3)CH2)y(CH2CH2O)n(CH2CH(CH3)O)yCH2CH(CH3)NR1R1, wherein n=1 to 100, R is H or CH3, each R1 independently is H or a substituted or unsubstituted C1-10 alkyl group (especially xe2x80x94CH2CH2xe2x80x94OH) and y=2 to 15.
The function of component iii) is to colour the polyurethane. Component iii) preferably comprises a chromophoric group and at least two functional groups selected from isocyanate and isocyanate-reactive groups. Preferably the colorant has two of said functional groups. Colorants having three of said functional groups may be present in low levels, preferably not exceeding 5% by weight relative to the weight of colorant having two of said functional groups.
The functional groups may be attached directly to the chromophoric group or may be attached through a linker group. An example of a suitable linker group is an alkylenediamine group attached to a chromophoric group via a triazine ring.
The chromophoric group preferably comprises an azo, anthraquinone, pyrroline, phthalocyanine, polymethine, aryl-carbonium, triphenodioxazine, diarylmethane, triarylmethane, anthraquinone, phthalocyanine, methine, polymethine, indoaniline, indophenol, stilbene, squarilium, aminoketone, xanthene, fluorone, acridene, quinolene, thiazole, azine, induline, nigrosine, oxazine, thiazine, indigoid, quinonioid, quinacridone, lactone, pyrroline, benzodifuranone or indolene group or a combination of two or more such groups. More preferred chromophoric groups are azo groups, especially monoazo, disazo, trisazo and phthalocyanine groups. Especially preferred monoazo groups comprise two aryl groups linked by an azo group where optionally one or both aryl groups and heteroaryl groups.
Preferably the colorant is a dye comprising a chromophoric group and at least two functional groups selected from isocyanate and isocyanate-reactive groups, especially a dye which is soluble in organic solvent.
Component iii) preferably comprises from 1 to 10, more preferably 1 to 5, especially 1, 2 or 3 of said dyes. The amount of component iii) used in the reaction mixture depends on the intensity of colour required in the polyurethane. The amount is preferably from 1 to 35%, more preferably from 10 to 30%, especially from 15 to 25% by weight relative to the total weight of components i), ii) and iii).
Optionally the mixture comprising components i), ii) and iii) further comprises a colourless isocyanate-reactive compound which is free from water-dispersing groups. Preferably such compounds are organic polyols or polyamines having a molecular weight up to 3000, more preferably up to 2000, especially from 400 to 2000. Preferred organic polyols include diols which are free from water-dispersing groups and mixtures thereof. Such diols may be members of any of the chemical classes of polymeric diols used or proposed to be used in polyurethane formulations. In particular, the diols may be polyesters, polyesteramides, polyethers (other than ones providing polyethyleneoxide and/or polypropyleneoxide groups), polythioethers, polycarbonates, polyacetals, polyolefins or polysiloxanes.
Further examples of optional diols which are free from water-dispersing groups include organic diols and polyols having molecular weights below 400. Examples of such lower molecular weight diols and polyols include ethylene glycol, diethylene glycol, tetraethylene glycol, bis(hydroxyethyl) terephthalate, cyclohexane dimethanol, furan dimethanol, polyethylene glycol and polypropylene glycol.
Terminating compounds may be used to cap off any excess isocyanate end groups in the colored water-dissipatable polyurethane resulting from the reaction of components i) ii) and iii), by the addition of compounds having an isocyanate-reactive group. Compounds having one isocyanate-reactive group include for example monoalcohols, monohydrazides, monoamines and monothiols. Compounds having two isocyanate-reactive may be used if one of the isocyanate-reactive groups is far more reactive than the other, thereby reacting essentially as a monofunctional terminating compound. For example in ethanolamine the amine group is far more reactive than the hydroxyl group. Optionally the terminating compound may be coloured. Examples of coloured terminating compounds include colorants as hereinbefore described, characterised in that they have only one isocyanate or isocyanate-reactive group.
The terminating compounds may also bear water-dispersing groups. If there are any excess isocyanate-reactive end groups in the coloured water-dissipatable polyurethane resulting from the reaction of components i) ii) and iii), these may optionally be capped off by the addition of compounds having one isocyanate group, for example alkyl monoisocyanates.
The polyurethane may contain further linkages in addition to urethane linkages, for example urea, amide, thiourea or thiourethane linkages. Preferably the coloured water-dissipatable polyurethane of the present invention (and resultant inks) is yellow, magenta, cyan or black.
The polyurethane may be prepared in a conventional manner by reacting the components having isocyanate groups with the components having isocyanate-reactive groups. Temperatures of from 30xc2x0 C. to 130xc2x0 C. are preferred and the reaction is continued until the reaction between the isocyanate groups and the isocyanate-reactive groups is substantially complete.
The relative amounts of components i), ii) and iii) are preferably selected such that the mole ratio of isocyanate groups to isocyanate-reactive groups is about 1.1 to 2:1, preferably from 1.2:1 to 2:1 and more preferably from 1.3:1 to 2:1. A two stage process may be utilised where a prepolymer is prepared either in solvent or as a melt, wherein the mole ratio of isocyanate groups to isocyanate-reactive groups from 1.3:1 to 2:1, preferably from 1.9:1 to 2:1, followed by reacting any excess isocyanate end groups with a terminating compound either in solvent or as a melt.
Alternatively a prepolymer may be prepared wherein the ratio of isocyanate groups to isocyanate-reactive groups from 1:1.1 to 1:2, preferably from 1:1.2 to 1:1.9. The excess isocyanate-reactive end groups may optionally be terminated with a terminating compound.
Preferably the polyurethane is not a chain-extended polyurethane. Preferably dissipation of the polyurethane in water results in essentially no chain-extension. In other words, preferably the reaction does not comprise a step in which two or more polyurethane molecules are deliberately linked together in water by a chain-extender, wherein the chain-extender comprises a compound having two or three isocyanate-reactive groups.
If desired a catalyst may be used to assist formation of the polyurethane. Suitable catalysts include butyl tin dilaurate, stannous octoate and tertiary amines as known in the art.
An organic solvent may optionally be included in the reaction mixture to lower its viscosity. Preferably a water-miscible solvent is used, for example N-methylpyrrolidone, dimethyl sulphoxide, dimethylformamide, a dialkyl ether of a glycol acetate methyl ethyl ketone or a mixture thereof.
Preferably the polyurethane according to the first aspect of the present invention has been obtained by the stated process.
Preferably the polyurethane (and any resultant inks) is yellow, magenta, cyan or black.
The polyurethane of the present invention may be purified if desired in the usual way for colorants used in ink jet printing inks. For example a mixture of the polyurethane and water may be purified by ion-exchange, filtration, reverse osmosis, dialysis, ultra-filtration or a combination thereof. In this way one may remove co-solvents used for the polymerisation, low molecular weight salts, impurities and free monomers.
In a second aspect of the present invention there is provided an ink comprising a coloured water-dissipatable polyurethane according to the first aspect of the present invention and a liquid medium. A preferred ink comprises:
(a) from 0.25 to 30 parts of a polyurethane according to the first aspect of the present invention; and
(b) up to 99.75 parts of a liquid medium;
wherein all parts are by weight and the number of parts of (a)+(b)=100.
The number of parts of component (a) is preferably from 0.5 to 28, more preferably from 2 to 25, and especially from 5 to 20 parts.
The inks according to the second aspect of the invention may be prepared by mixing the polyurethane with the liquid medium. Suitable techniques are well known in the art, for example agitation, ultrasonication or stirring of the mixture. The mixture of the polyurethane and liquid medium may be in the form of a dispersion, emulsification, suspension, solution or mixture thereof.
Preferably the polyurethane is mixed with a first liquid medium, followed by mixing the resultant mixture with a second liquid medium.
The liquid medium is preferably water, a mixture of water and an organic solvent and an organic solvent free from water. For example the polyurethane may be added to water followed by the addition of one or more organic solvents. Preferably the first liquid medium is an organic solvent and the second liquid medium is water and a mixture of water and one or more organic solvents.
When the liquid medium comprises a mixture of water and an organic solvent, the weight ratio of water to organic solvent is preferably from 99:1 to 1:99, more preferably from 99:1 to 50:50 and especially from 95:5 to 60:40.
It is preferred that the organic solvent present in the mixture of water and organic solvent is a water-miscible organic solvent or a mixture of such solvents. Preferred water-miscible organic solvents include C1-6-alkanols, preferably methanol, ethanol, n-propanol, isopropanol, n-butanol, sec-butanol, tert-butanol, n-pentanol, cyclopentanol and cyclohexanol; linear amides, preferably dimethylformamide or dimethylacetamide; ketones and ketone-alcohols, preferably acetone, methyl ether ketone, cyclohexanone and diacetone alcohol; water-miscible ethers, preferably tetrahydrofuran and dioxane; diols, preferably diols having from 2 to 12 carbon atoms, for example pentane-1,5-diol, ethylene glycol, propylene glycol, butylene glycol, pentylene glycol, hexylene glycol and thiodiglycol and oligo- and poly-alkyleneglycols, preferably diethylene glycol, triethylene glycol, polyethylene glycol and polypropylene glycol; triols, preferably glycerol and 1,2,6-hexanetriol; mono-C1-4-alkyl ethers of diols, preferably mono-C1-4-alkyl ethers of diols having 2 to 12 carbon atoms, especially 2-methoxyethanol, 2-(2-methoxyethoxy)ethanol, 2-(2-ethoxyethoxy)-ethanol, 2-[2-(2-methoxyethoxy)ethoxy]ethanol, 2-[2-(2-ethoxyethoxy)-ethoxy]-ethanol and ethyleneglycol monoallylether; cyclic amides, preferably 2-pyrrolidone, N-methyl-2-pyrrolidone, N-ethyl-2-pyrrolidone, caprolactam and 1,3dimethylimidazolidone; cyclic esters, preferably caprolactone; sulphoxides, preferably dimethyl sulphoxide and sulpholane. Preferably the liquid medium comprises water and 2 or more, especially from 2 to 8, water-soluble organic solvents.
Especially preferred water-soluble organic solvents are cyclic-amides, especially 2-pyrrolidone, N-methyl-pyrrolidone and N-ethyl-pyrrolidone; diols, especially 1,5-pentane diol, ethyleneglycol, thiodiglycol, diethyleneglycol and triethyleneglycol; and mono-C1-4-alkyl and C1-4-alkyl ethers of diols, more preferably mono-C1-4-alkyl ethers of diols having 2 to 12 carbon atoms, especially 2-methoxy-2-ethoxy-2-ethoxyethanol.
When the liquid medium comprises an organic solvent free from water, (i.e. less than 1% water by weight) the solvent preferably has a boiling point of from 30xc2x0 to 200xc2x0 C., more preferably of from 40xc2x0 to 150xc2x0 C., especially from 50 to 125xc2x0 C. The organic solvent may be water-immiscible, water-miscible or a mixture of such solvents. Preferred water-miscible organic solvents are any of the hereinbefore described water-miscible organic solvents and mixtures thereof. Preferred water-immiscible solvents include, for example, aliphatic hydrocarbons; esters, preferably ethyl acetate; chlorinated hydrocarbons, preferably CH2Cl2; and ethers, preferably diethyl ether; and mixtures thereof.
When the liquid medium comprises a water-immiscible organic solvent, preferably a polar solvent is included because this enhances solubility of the polyurethane in the liquid medium. Examples of polar solvents include C1-4-alcohols. In view of the foregoing preferences it is especially preferred that where the liquid medium is an organic solvent free from water it comprises a ketone (especially methyl ethyl ketone) and or an alcohol (especially a C1-4-alkanol, more especially ethanol or propanol).
The organic solvent free from water may be a single organic solvent or a mixture of two or more organic solvents. It is preferred that when the medium is an organic solvent free from water it is a mixture of 2 to 5 different organic solvents. This allows a medium to be selected which gives good control over the drying characteristics and storage stability of the ink.
Ink media comprising an organic solvent free from water are particularly useful where fast drying times are required and particularly when printing onto hydrophobic and non-absorbent substrates, for example plastics, metal and glass.
A preferred liquid medium comprises:
(a) from 75 to 95 parts water; and
(b) from 25 to 5 parts in total of one or more solvents selected from diethylene glycol, 2-pyrrolidone, thiodiglycol, N-methylpyrrolidone, cyclohexanol, caprolactone, caprolactam and pentane-1,5-diol;
wherein the parts are by weight and the sum of the parts (a) and (b)=100.
The ink may also contain a surfactant. This helps to dissipate the polyurethane in addition to the dissipation caused by dispersing groups provided by component ii) of the polyurethane. Optionally the ink may also contain other ingredients used in ink jet printing inks, for example conductivity agents, defoamers, anti-oxidants, corrosion inhibitors, bacteriocides and viscosity modifiers.
The ink preferably has a pH of from 3 to 11, more preferably of from 4 to 10. Such a pH may be obtained by the addition of a base, acid or a pH buffer. Where a base is used this is preferably the same base as was used to neutralise the anionic dispersing group during the preparation of the coloured water-dissipatable polyurethane.
The viscosity of the ink is preferably less than 20 cp, more preferably less than 10 cp, at 20xc2x0 C.
Preferably the ink has been filtered through a filter having a mean pore size below 10 xcexcM, preferably below 5 xcexcM, more preferably below 2 xcexcM especially of 0.45 xcexcM. In this way particulate matter is removed which could otherwise block the fine nozzle in an ink jet printer.
The inks of the second aspect of the present invention have the advantage that they are suitable not only for the use of piezoelectric ink jet printers but also for the use of thermal and continuous ink jet printers.
Inks of the second aspect of the present invention form discrete droplets on the substrate with little tendency for diffusing. Consequently sharp images with excellent print quality and little if any bleed between colours printed side by side can be obtained. Furthermore the inks show good storage stability, wet and light fastness and fastness to both acidic and alkaline highlighter pens.
A further aspect of the invention provides a process for printing an image on a substrate comprising applying thereto an ink containing a coloured water-dissipatable polyurethane of the present invention by means of an ink jet printer.
The ink used in this process is preferably as defined in the second aspect of the present invention.
The ink jet printer preferably applies the ink to the substrate in the form of droplets which are ejected through a small nozzle onto the substrate. Preferred ink jet printers are piezoelectric ink jet printers and thermal ink jet printers. In thermal ink jet printers, programmed pulses of heat are applied to the ink in a reservoir by means of a resistor adjacent to the nozzle, thereby causing the ink to be ejected in the form of small droplets directed towards the substrate during relative movement between the substrate and the nozzle. In piezoelectric ink jet printers the oscillation of a small crystal causes ejection of the ink from the nozzle.
The substrate is preferably paper, plastic, a textile, metal or glass, more preferably paper, an overhead projector slide or a textile material, especially paper.
Preferred papers are plain, coated or treated papers which may have an acid, alkaline or neutral character. Most preferably the substrate is a plain or coated paper.
According to a further feature of the invention there is provided an ink jet printer cartridge containing an ink as hereinbefore defined.